Echo cancellation circuits which are well known in the telephone communications environment usually employ a form of adaptive finite impulse response digital filter. In a typical system the echo suppression apparatus at the near subscriber functions to disable the outgoing path from that subscriber when signals from the far end subscriber appear on the incoming path. Echoes due to incoming signals on the receive path are, therefore, prevented from returning to the far end subscriber over the outgoing path. Double talking refers to the condition when the near end subscriber breaks in and both subscribers are talking simultaneously. Prior art echo suppressors include double talk detectors which distinguish between speech signals generated on the outgoing path by the near end subscriber (double talk signals) and echo signals returning on the outgoing path due to the far end subscriber speech signals on the incoming path. If the outgoing path signal exceeds the incoming path signal it is assumed that the near end subscriber is transmitting and the echo suppression is disabled. When the opposite condition occurs it is assumed that the near end subscriber is not transmitting and the echo is suppressed.
The patents to May, Jr. (U.S. Pat. No. 3,973,086), Helder (U.S. Pat. No. 3,992,594) and Geigel et al. (U.S. Pat. No. 4,029,912) discuss the problem of distinguishing between an echo and double talk. The above systems work by initially providing an approximated echo using a pair of storage memories, one following decreasing magnitude signals to give an output corresponding to the anticipated (or worst case) echo end delay, while the other memory is used to approximate echo as long as its output is less than the output of the first memory. Otherwise, the first memory is used for the echo approximation. The second memory follows increasing magnitude signals, less a certain transmission loss, and holds the value of the last signal peak while the signal magnitude is decreasing. These approximations are compared with the actual signals on the echo return path. Double talk detection by this method is evidenced in all three of the above-mentioned patents. This method is different than that of the subject disclosure in which a third detector is used to control the signal path through the other detectors during initial adaptive filtering when the residual echo is less than the true echo.
The patent to Araseki et al. (U.S. Pat. No. 4,005,277) describes an echo controller comprising a mode switch for switching an echo suppressor and canceller. This switch normally supplies the echo cancelled signal to the outgoing path and further evaluates characteristics of the echo path and operation of the echo canceller to substitute the echo suppressed signal for the echo cancelled signal only when the path characteristics are questionable. The mode switch thus monitors the double talk and carries out the switching between the echo cancelled and suppressed signals in the absence of double talk.
Other patents to Araseki et al. (U.S. Pat. No. 4,012,603) and Ochiai et al. (U.S. Pat. No. 3,787,645) discuss various aspects of this type of double talk detection system.
A "Twelve-Channel Digital Echo Canceller" which uses a digital filter and a center clipper is described by D. Duttweiler in IEEE Transactions on Communications, Vol. Com-26, No. 5, May 1978.
The prior art, such as that discussed above, is further illustrated in FIG. 1, which shows an echo canceller circuit at the near talker location. A received signal from the distant talker Dt is passed through insulating amplifier 7 to the receiver output Ro. Path 2 represents the arbitrary path taken by the received signal causing an echo thereof to be fed back into the transmitting portion of near talker equipment. The echo signal Ec is similar to the signal Dt but attenuated by the propagating medium 2.
The use of adaptive filter 1 for echo cancellation in long distance telephone circuits is well known and described in detail by S. J. Campanella et al., "ANALYSIS OF AN ADAPTIVE IMPULSE RESPONSE ECHO CANCELLER", Comsat Technical Review, Vol. 2, No. 1, Spring 1972, pp. 1-36, and by O. A. Horna, "ECHO CANCELLER UTILIZING PSUEDO LOGARITHMIC COATING", NTC 1977 Conference Record, Vol. 1, pp. 04:7-1 through 04:7-8. Such adaptive filters compute an estimate E'c of the true echo signal Ec and apply it to subtractor circuit 3. After an initial adaptive period, the proper transfer characteristic is developed in filter 1 and only a small residual echo signal Re remains at the output subtractor 3. The residual Re is then easily suppressed by a non-linear center clipper circuit.
Both adaptive filter 1 and center clipper 5 may remain in an active state as long as desired to effect cancellation of the echo signal Ec. However, in the event that the near talker breaks into the conversation, a "double talk" condition occurs. Both the echo Ec and the near talker speech Nt appear at the input to the transmitting portion of the near talker equipment. Under these conditions, two changes must take place: (1) the adaptation process of the filter 1 must be "frozen" so that it will not be contaminated by the uncorrelated near talker signal Nt and (2) the center clipper 5 must be disabled in order not to distort the near talker speech. The residual echo signal Re, being much smaller than Nt is completely masked by the speech signal.
The function of the circuit in FIG. 1 is based on the assumption that during double talk, the volume of near talker speech Nt is higher than the volume of the echo, i.e., Nt is greater than Ec. The tripping point of double talk detector 6 can be adjusted to indicate a double talk condition when, EQU Si=Ec+Nt&gt;0.5Dt.
Detector 6 under these conditions disables the center clipper 5 and the correction loop of filter 1 over lines 9 and 8, respectively.
Certain conditions will degrade performance of the latter echo cancellation circuit. One such condition, known as false double talk, can occur when the distant speaker pauses. The echo, delayed due to the propagation delays inherent in path 2, will be greater than 0.5 Dt thereby causing the double talk detector to falsely indicate a double talk condition. Center clipper 5 will be turned off resulting in a burst of residual echo Re on the send-out line.
The opposite situation exists when the near talker speech is of a lower amplitude than the echo signal. This can occur in low quality, long two-wire circuits. In this case, detector 6 is unable to detect the double talk condition and the impulse response of filter 1 is contaminated and the center clipper distorts the speech signal Nt.